Fish teeth:

ToadstonesToadstones are the button-shaped fossil teeth of the fish Lepidotes. In folklore they were once thought to have come from the heads of living toads.

Toads were of particular interest to medical practitioners during the seventeenth century (Baldwin 1993).

It was believed that concoctions made from pulverised toads provided protection against bubonic plague if placed in amulets and worn around the neck or wrists. This was justified on several grounds, notably that the toad's warty skin resembled the carbuncles and spots developed by plague victims.